1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
Conventionally, a projection exposure apparatus has been widely used, which projects a circuit pattern drawn on a reticle onto a wafer or the like as a substrate by using a projection optical system and transfers the circuit pattern onto the wafer.
Recently, with demands for miniaturization of semiconductor devices, higher resolutions have been required for projection optical systems. In order to meet this requirement, the wavelength of exposure light may be decreased, and the numerical aperture (NA) of a projection optical system may be increased. An immersion projection optical system has also been proposed, which can achieve an NA of 1 or more by a so-called immersion technique of filling the space between the final lens (final surface) of a projection optical system and a wafer with a liquid. To provide a projection optical system having extremely high performance, requirements for extreme reductions in various aberrations as well as increases in resolution have become severer.
An exposure apparatus including a projection optical system is required to satisfy both the above requirements for high optical performance and miniaturization. An increase in the aperture of a glass material with an increase in NA has become a large factor that increases the cost of an exposure apparatus.
Satisfying both the requirements for an increase in the NA of a projection optical system and exposure apparatus and their miniaturization will increase the refracting power of each lens constituting the projection optical system. As the refracting power of each lens increases, a light beam is strongly bent or bounced. This causes a large difference in pupil shape between on-axis light and off-axis light, resulting in an increase in so-called pupil aberration.
As pupil aberrations, especially the curvature of field of the pupil and the comatic aberration of the pupil increase, the image-side telecentric characteristic of a projection optical system deteriorates, and image distortion in the exposure apparatus increases. In addition, variations in NA (ΔNA) between image heights relative to the desired NA of the projection optical system increase. As a result, it becomes impossible to maintain the high uniformity of the set NA values of the exposure apparatus.
As a measure against a deterioration in image telecentric characteristic due to the influence of pupil aberration, a technique has been proposed, which positions two variable aperture stops at a plurality of positions near the pupil position in a projection exposure apparatus including a projection optical system having an NA of about 0.75. This proposal is disclosed in, for example, Japanese Patent Laid-Open No. 2002-118053.
Furthermore, recently, with an increase in the NA of a projection exposure apparatus, an exposure apparatus including a projection optical system of a plural-times imaging system has been proposed. According to this proposal, a variable aperture stop is positioned in each imaging optical system of the plural-times imaging system, that is, at each pupil position (see, for example, Japanese Patent No. 3690819 and Japanese Patent Laid-Open No. 2007-043168).
The optical system disclosed in Japanese Patent Laid-Open No. 2002-118053 is a dioptric (refractive) projection optical system of a one-time imaging system with a numerical aperture (NA) of 0.75. This optical system achieves good optical performance in a desired NA area by adjusting the upper-side light and lower-side light of a light beam using two aperture stops positioned near the pupil position.
When, however, both the requirements for a further increase in NA and miniaturization are to be satisfied, the refracting power corresponding to the maximum lens diameter increases, the curvature of the pupil is also positioned on the wafer side. For this reason, the arrangement comprising the two aperture stops described in Japanese Patent Laid-Open No. 2002-118053 cannot achieve a desired image-side telecentric characteristic, and the inter-image-height NA variations also increase. Furthermore, there is no description about the range of the numerical aperture determined by the two aperture stops.
The optical system disclosed in Japanese Patent No. 3690819 is a catadioptric projection optical system of a two-times imaging system. The optical system disclosed in Japanese Patent Laid-Open No. 2007-043168 is a dioptric projection optical system of a two-times imaging system. These references disclose a technique of achieving a desired image-side telecentric characteristic and inter-image-height NA uniformity in both the optical systems by positioning a variable aperture stop at the pupil position in each imaging optical system.
In order to form the aperture stop mechanisms of a projection optical system and exposure apparatus more simply, an aperture stop is preferably positioned near one pupil position regardless of the number of times of imaging. In general, positioning stops at a plurality of pupil positions while maintaining miniaturization is not preferable for correcting aberrations. Furthermore, it is not preferable to position a stop near a pupil position other than that of the final imaging system. This is because, since the tilt of a light beam passing through the aperture stop increases, aberration sensitivity generally increases due to the errors of the diameter and position of the aperture stop.